This is a worldwide taxonomic revision of the genus Yllenus, with 65 valid species being diagnosed, figured and (re)described. A key to three species groups and distribution maps for all revised species are also provided. This work is intended for arachnologists, mainly taxonomists, as well as for biogeographers, local biologists and naturalists. Contents include: Introduction, Materials and methods, Terminology, Acknowledgements, Genus Yllenus Simon 1868, Definition, Diagnosis and affinities, Composition, Distribution, Natural history, Key to the species groups of Yllenus, Survey of species, The albocinctus species group, The arenarius species group, The hamifer species group, Doubtful and invalid species names, References, Index to species.